This invention relates to an improved machine tool, and improved methods of running a machine tool.
Machine tools can be used to fashion objects from blocks of material by removing excess material in a predetermined manner to arrive at the desired shape. This fashioning generally involves two processes: bulk material removal, or roughing, and fine working of the object. Bulk material removal is designed to remove excess material as quickly and as efficiently as possible to allow the fine working of the object to take place. Fine working of the object is a much finer process in which material removal is slower, more precise, and consequently the surface finish is higher.
To allow material to be removed a path must be planned for the material-remover of the machine tool. Traditionally this was performed by the manual control of a skilled operator. In the last few decades computer numerically controlled (CC) machine tools have become widespread and a set of instructions, or computer program, is used to control the path of the material-remover of the machine tool in such machines.
The set of instructions governing the path determines the efficiency of the bulk material removal process. It is desired to remove as much material in as little time as possible, but it is imperative that the material from which the object will be fashioned is not damaged.
Various material removal methods are well known and FIGS. 1 and 2 show examples of two such methods. FIG. 1 shows perhaps the simplest approach in which material 1 around the object 2 is removed by raster scanning the machine tool. In each pass of the machine tool the material-remover will remove an amount of material 1, and the line 4 within the material 1 shows the centre line of each cut. It will be appreciated that this method is inefficient since the cutting tip does not run continuously, but must be stopped and moved around the object 2 in order to complete the cut on the opposite side of the object.
FIG. 2 shows another prior art method of performing the bulk material removal. Again, the line within the material 1 shows the path, or the centre line of a cut, performed by the cutting tip. The cutting tip makes a series of passes through the material 1 until only the object 2 remains.
However, none of the prior art methods discussed herein remove bulk material as efficiently as may be desired.
It is an object of the invention to ameliorate the problems of the prior art.
According to a first aspect of the invention there is provided a machine tool having a material-remover, the material-remover being able to move in at least two degrees of freedom and at a particular instant being arranged to remove an amount of material from material that it is processing, movement of said material-remover being under the control of processing circuitry, said processing circuitry determining a path along which the material-remover should move, and in determining said path allows the depth of a cut made by the material-remover to vary.
An advantage of such a machine is that it is likely to be more efficient than bulk material removal than prior art machine tools. Prior art machines are arranged to determine the path of a machine tool around the object such that the material-remover was constrained to remove roughly constant amount of material at any particular instant.
Prior art cutting machines remove slices of constant width from the material until the material has been removed as desired. This is as discussed in relation to FIGS. 1 and 2 above. The applicants have realised that the prior art machines are sub-optimal because the amount of material removed on each cut is constant and that therefore, the solution for the optimum material-remover path is over constrained.
The machine tool may be any one of the following machines: a milling machine; a machining centre, a multi-axis machining centre, a lathe.
Conveniently, the material-remover of the machine tool is arranged to rotate about an axis. Such an arrangement is convenient because it facilitates material removal.
Preferably, the processing circuitry is arranged to attempt to move the material-remover such that the magnitude of its velocity is roughly constant. Such an arrangement is advantageous because it is more efficient to run a machine tool at its maximum cutting speed rather than continuously accelerating and decelerating the material-remover and can result in an improved life of the material remover.
However, the processing circuitry may be arranged to vary the speed of the material-remover for some complex cutting paths. It will be appreciated that the path of the material-remover will have a minimum cutting radius, which is a function of the speed of the material-remover and determined by the maximum axis acceleration of the machine.
Therefore, the processing circuitry may be arranged to reduce the speed of the material-remover in order to achieve smaller cutting radii than are possible at the target material-remover speed.
A material-remover has a maximum amount of material that it can remove in a single cut. The minimum amount of material that it can remove in a single cut is zero. The processing circuitry may comprise a track planner arranged to associate one or more tracks around the perimeter of an object to be machined, the or each track comprising a locus of all the possible material remover paths around the object. An advantage of such a track is that it provides a convenient starting point from which to calculate a material-remover path.
Alternatively, or additionally, the processing circuitry may comprise a track planner arranged to associate one or more contours around the perimeter of an object to be machined. In such an embodiment it is preferred to select a contour comprising the centre line of the track (although any value of contour between the zero and the maximum depth of cut is possible).
Conveniently, the track planner is arranged to associate a series of such tracks and/or contours around the perimeter of the object to be fabricated. As such, a series of tracks and/or contours are built up, each providing an indication of possible material-remover paths. Of course, whilst the first track and/or contour is based upon the perimeter of the object to be fabricated, subsequent tracks and /or contours are based upon the previous track and/or contour.
The track planner may produce tracks that are of variable width.
The processing circuitry may further comprise a node associator arranged to associate a number of nodes with predetermined points around the track and/or contour that has been calculated.
Preferably, the node associator is arranged to associate points with corners of the track and/or contour. A corner in the track and/or contour may be defined as any deviation in the track/contour from a straight line that results in a turn tighter than the minimum cutting radius of the material remover. Therefore, if the track/contour incorporates a curve having a radius greater than the minimum turning radius of the material remover no node may be associated therewith.
The node associator may be arranged to associate predetermined nodes with the inside of the track. Additionally the node associator may be arranged to associate other predetermined nodes with the outside to the track.
Conveniently, the node associator is arranged to associate nodes associated with the inside of the track with convex corners of the object to be machined. The node associator may be arranged to associate nodes associated with the outside of the track with concave corners of the object to be machined.
Conveniently, the processing circuitry further comprises a curve associator arranged to associate a curve with each of the nodes produced by the node associator. Preferably, the curve associated with the node by the curve associator has a radius corresponding to the minimum radius of a path of the material-remover of the machine tool. Such an arrangement ensures that it is possible to arrange the path for the material-remover such that it does not result in the velocity of the material remover being reduced because a turn tighter than the minimum cutting radius at its maximum velocity has been specified.
The curve associator may be arranged to determine whether a curve that it is associating with a node produced by the node associator is less efficient than a straight line path, and if this is the case to replace the curve with a straight line path. The skilled person will appreciate that the material remover has momentum, such that small deviations in a path may be smoothed out, and that processing complex paths requires processing power. Therefore, it may be possible to make a path less efficient by introducing curves that attempt to cause the material remover to deviate from a straight line by small amounts whilst increasing the processing required. Therefore, causing the curve associator to assess the efficiency of the curve may be advantageous because it helps to increase the efficiency of the path along which the material remover moves.
The curve associator may be arranged to reduce the radius of one or more curves if curves centred on opposite side of the track intersect one another to block the track. Reducing the radius of the curves in this manner results in a reduction in the cutting speed, but may allow a path to be plotted whereas otherwise, no path may have been possible.
The curve associator may be arranged to associate more than one node with any one curve. Such an arrangement is advantageous in some circumstances (particularly when nodes occur close to one another) because it can remove the need for a number of curves and thus the complexity of the solution may be reduced.
Conveniently, the curves are situated at the nodes generated by the node associator such that the radius of the curve passes close to the node and preferably, the curve is centred on the bisector of the corner. It is convenient for the curve associator to associate curves with the nodes in this manner since this provides a position that is computationally simple to find. Calculation problems may occur if the curve passes inside the node and ensuring that the curve passes close to the node, rather than touching the node provides a safety margin helping the ensure that the curve never passes inside the node.
The curve associator may alter the position of the curve such that it does not lie on the bisector of the corner. This can be advantageous if it allows a curve of larger radius to be associated with the node.
The curve associator may be arranged to associate circles, or portions of circles with the nodes. Circles are convenient since they are mathematically simple, but any tangent continuous curve will suffice. It is preferable that the curvature of the curve is continuous, but not essential.
Preferably, the processing circuitry further comprises a tangent generator arranged to associate a path between each of the curves generated by the curve associator, the path being a tangent to each of the curves that it contacts. An advantage of the tangent generator is that the path so produced is the shortest line around the track produced by the track planner without passing inside the curves generated by the curve associator.
The processing circuitry is preferably arranged to convert the tangents generated by the tangent generator together with portions of the curves provided by the curve associator into a path for the material-remover.
Conveniently, the processing circuitry is arranged to generate paths that form a closed loop around the object to be fabricated. Such an arrangement is advantageous because it completely specifies a path around the object. However, in other embodiments the processing circuitry may be arranged to generate paths that are open and do not form a closed loop around the object to be fabricated.
Preferably the processing circuitry is arranged to produce a series of paths such that an object can fabricated from a block of material. In some embodiments the processing circuitry is arranged to produce linking paths for the material remover linking the series of paths previously calculated. The linking paths may be curves, which run one path into the next. Such linking paths are advantageous because they further help to improve the efficiency of the material removal.
According to a second aspect of the invention there is provided a method of removing material from a block of material surrounding an object to be fashioned, said method comprising plotting a path for a material-remover of a machine tool, said path being optimised by allowing the depth of a cut made by the material-remover to vary.
An advantage of such a method is that it is likely to be more efficient at bulk material removal than prior art methods. Prior art methods are arranged to determine the path of a machine tool around the object to be fashioned such that the material-remover cuts to a substantially constant depth.
Preferably, the method comprises attempting to move the material-remover at roughly a constant speed. Such a method may prove more efficient than continuously accelerating and decelerating the speed of the material-remover and may increase the life of the material-remover.
Conveniently, the method allows the speed of the material-remover to be varied if its path includes a turn requiring an acceleration beyond that which the machine is capable of providing.
The method may comprise calculating a first track around the perimeter of an object to be machined. Said track may occur between the minimum and maximum amounts of material that can be removed in a single cut by the material-remover and comprise the locus of all possible material-remover paths between these minimum and maximum amounts.
Alternatively, or additionally, the method may comprise calculating a contour around the perimeter of an object to be machined, displaced from the object by a predetermined amount.
If a contour is calculated it is preferable that it corresponds to the centre line of a track comprising the locus of all material-remover paths around the perimeter of the object. Thus, the contour corresponds to a path for the material remover corresponding to a cut to a depth of 50% of its maximum in single pass. The contour may however, correspond to a depth of cut for the material-remover at any value from zero to its maximum.
A plurality of tracks and/or contours may be built up around the perimeter of the object to be fabricated, preferably with each successive track and/or contour occurring at a distance corresponding to the maximum depth of material that can be removed by the material-remover in a single cut. Therefore, a series of tracks and/or contours are built up, each providing an indication of the material that it is possible for the material-remover to remove in a series of passes.
Preferably the method comprises associating a number of nodes with predetermined points around the track and/or contour previously calculated. Preferably, each node occurs at points coincident with corners of the track and/or contour. A corner in the track may be defined as any deviation in the track from a straight line that results in a turn tighter than the minimum cutting radius of the material-remover.
Conveniently, predetermined nodes are associated with the inside of the track previously determined. Additionally predetermined nodes may be associated with the outside to the track previously determined.
Nodes associated with the inside of the track may be associated with convex corners of the object to be machined. Nodes associated with the outside of the track may be associated with concave corners of the object to be machined.
A curve may be associated with each of the nodes generated by the method. Preferably, each curve has a radius corresponding to the minimum cutting radius of the material-remover of the machine tool.
Such a method can be used to ensure that the path for the material-remover is such that it does not enter inside the minimum cutting radii associated with a corner of the object to be fabricated and therefore, cause the material-remover to slow. It will be appreciated that it is possible to reduce the minimum cutting radius by slowing the speed of the material-remover, although it is desirable not to alter the velocity of the material-remover.
Conveniently, the method arranges the curves at the nodes such that the radius of the curve passes close to the node and preferably, the curve is centred on the bisector of the corner. It is convenient for the curve to be positioned in such a manner since this provides a position that is computationally simple to find.
Further, the method comprises plotting a path comprising a tangent between each of the curves, together with a portion of one or more of the curves. An advantage of such tangents is that the path so produced is the shortest line around the track without passing inside the curves corresponding to the minimum cutting radii.
The method may comprise reducing the radii of curves associated with the nodes if the curve extends beyond the track. Such a method helps to ensure that the path is possible.
Conveniently, the method generates one or more paths that forms closed loops around the object to be fabricated. Such an arrangement is advantageous because it completely specifies a path around the object. However, in other embodiments the method generates one or more paths that are open and do not form a closed loop around the object to be fabricated.
According to a third aspect of the invention there is provided a method of plotting a path for the material-remover of a machine tool, wherein said method comprises allowing the depth of a cut performed by the material-remover to vary.
The method of the third aspect of the invention may take any of the features from the second aspects of the invention.
According to a fourth aspect of the invention there is provided a computer readable medium coded with instructions to cause a computer to perform the method according to the second aspect of the invention.
According to a fifth aspect of the invention there is provided a computer readable medium coded with instructions to cause a computer to perform the method according to the third aspect of the invention.
The computer readable medium of the fourth or fifth aspects of the invention may comprise a disk, a ZIP disk, an LS120 disk, a CDROM, a DVD ROM/RAM, any other form of magneto and/or optical storage, or may comprise an email, an internet download, or any other transmitted signal.